The present invention relates to a prosthesis which functionally substitutes for the hand of a person who has had portions of a hand amputated as a result of disease or injury or who because of paralysis of muscles controlling the movement of digits is unable to grasp objects effectively. Generally, such prostheses are in one of two classes--those which are primarily directed toward a cosmetic imitation of a natural hand and those which are primarily directed toward providing the functions of the missing or paralyzed members. The goals of the two classes of prostheses are generally incompatible although many devices have been developed which seek to reconcile these goals.
An example of a device which is primarily cosmetic is shown in U.S. Pat. No. 1,893,714 in which finger elements and thumb elements shaped to imitate flexed fingers and thumb are adapted to fit over the stubs of amputated fingers and thumb. Both elements are secured by a wrist band. The normal movement of the finger and thumb stubs causes similar movement of the artificial elements to provide a crude grasping action which can engage small objects such as a pen or pencil.
Other devices utilize cable and pulley systems to flex multi-jointed artificial finger or finger and thumb elements. These are more functional than the device in U.S. Pat. No. 1,893,714 and the artificial members may cosmetically resemble natural appendages. But one disadvantage of these devices is the lack of durability and strength of the cables and the possibility of breakage or disconnection of the cables. U.S. Pat. No. 4,291,421 shows a hand and forearm prosthesis actuated by cables anchored on the upper arm. U.S. Pat. No. 2,867,819 shows artificial fingers actuated by cables anchored on the wrist for a person whose wrist and thumb is intact. U.S. Pat. No. 2,893,016 and No. 2,553,277 use cables to move paralyzed digits.
Many devices have been proposed to provide a grasping function when a hand has been completely amputated. See, for example, U.S. Pat. No. 1,546,726 and No. 4,016,607, both of which use interacting hooks.
Many devices have also been proposed to provide a grasping function when a portion of the fingers, thumb or metacarpal region remains. U.S. Pat. No. 1,498,029 and No. 3,434,163 both disclose artificial fingers fixedly anchored at the wrist for use with a thumb which remains and is movable to provide a grasping function for small objects. U.S. Pat. No. 2,568,298 discloses articulated artificial fingers which can assume an open or closed relationship with a thumb. This is accomplished using spring loaded levers and a change of position must be initiated by pressure from the individual's intact hand or other object.
U.S. Pat. No. 4,021,866, designed for use where only the wrist joint and finger stubs remain, discloses one plate anchored proximally to or above the wrist and hinged adjacent the wrist to a second plate. Flexing and extension of the wrist joint opens and closes the space between the two plates.
All of these devices have a common functional deficiency. They are adapted to close upon and grasp an object in a narrow range of dimensions. An operating hand prosthesis designed to grasp small objects, such as a needle, pen or pencil or intravenous tubing, is not well adapted to grasping larger articles such as a doorknob, tumbler of liquid, computer mouse, or pieces of furniture or equipment, and the reverse is also true. An intact hand, by contrast, has a range of grasp from the almost dimensionless, the thickness of a hair, to five to seven or more inches.
Most of the devices have an outside or largest range of grasp defined by the distance between the end of the thumb in its extended position and the ends of the artificial fingers in their extended position. In the normal hand a greater range can be obtained by flexing the wrist and fingers posteriorly while maintaining the thumb in extension. The device in U.S. Pat. No. 4,021,866 conceivably could provide a broader range limited only by the length of the grasping plates. However, in this device the hinge produces grasping surfaces which diverge from one another in a "V" shape which is antithetical to secure grasping because the grasping force tends to move the grasped object toward the ends of the plates.